Hybrid work vehicle

ABSTRACT

A hybrid work vehicle includes a work device, a traveling drive device, a lever operation quantity detection unit, an engagement state detection unit, a pedal operation quantity detection unit, a travel state detection unit, and an engine control unit. The engine control unit controls an engine rotation rate based upon at least either the lever operation quantity or the pedal operation quantity in correspondence to either the engaged state or the non-engaged state and either the traveling state or the non-traveling state.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/378,760, filed on Aug. 14, 2014, which is a National Stage ofPCT/JP2013/053570, filed on Feb. 14, 2013, which claims priority fromJapanese Patent Application No. 2012-031149, filed on Feb. 15, 2012, thedisclosures of which are expressly incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a hybrid work vehicle.

BACKGROUND ART

There is a wheel loader known in the related art equipped with atraveling drive device that transmits the rotation of an engine to thewheels via a torque converter (see patent literature 1). The wheelloader described in patent literature 1 is a work vehicle, the enginerotation rate of which is controlled in correspondence to the extent ofaccelerator pedal operation.

CITATION LIST Patent Literature

Patent literature 1: International Publication WO2010/147232

SUMMARY OF INVENTION Technical Problem

In a work vehicle such as a wheel loader, a work device or a travelingdrive device may be operated alone or they may be operated incombination as the work vehicle is engaged in various types of workoperations including excavation, loading and traveling operation. In awork vehicle equipped with a torque converter in the related art, suchas the work vehicle disclosed in patent literature 1, the traveling loadand the work load are borne directly by the engine, and for this reason,the engine is subjected to significant load fluctuations. Since an amplemargin must be allowed for torque in consideration of such loadfluctuations, a problem arises in that the engine rotation rate ishigher when there is no work load, compared to the engine rotation ratecorresponding to the traveling load.

Solution to Problem

According to the first aspect of the present invention, a hybrid workvehicle comprises: a work device that is driven with pressure oilprovided from a hydraulic pump driven by an engine; a traveling drivedevice that is driven by a traveling motor driven with electric power; alever operation quantity detection unit that detects a lever operationquantity of a control lever operated to control the work device; anengagement state detection unit that detects an engaged state and anon-engaged state of the work device; a pedal operation quantitydetection unit that detects a pedal operation quantity of an acceleratorpedal; a travel state detection unit that detects a traveling state anda non-traveling state of the traveling drive device; and an enginecontrol unit that controls the engine rotation rate based upon at leasteither the lever operation quantity or the pedal operation quantity incorrespondence to whether the engagement state detection unit hasdetected the engaged state or the non-engaged state and whether thetravel state detection unit has detected the traveling state or thenon-traveling state.

According to the second aspect of the present invention, in the hybridwork vehicle according to the first aspect, the hybrid work vehiclefurther comprises: a first rotating electric machine that is driven bythe engine and generates first AC power; a first power conversion unitthat converts the first AC power, generated by the first rotatingelectric machine, to first DC power; a power storage unit that outputssecond DC power; a second power conversion unit that converts at leastone of the first DC power resulting from conversion by the first powerconversion unit, and the second DC power output from the power storageunit, to second AC power; and a second rotating electric machineconfiguring the traveling motor, which is driven with the second ACpower resulting from conversion by the second power conversion unit.

According to the third aspect of the present invention, in the hybridwork vehicle according to the first or the second aspect, when theengagement state detection unit detects the non-engaged state and thetravel state detection unit detects the traveling state, the enginecontrol unit controls the engine rotation rate based upon the pedaloperation quantity; and, when the engagement state detection unitdetects the engaged state and the travel state detection unit detectsthe non-traveling state, the engine control unit controls the enginerotation rate based upon the lever operation quantity.

According to the fourth aspect of the present invention, in the hybridwork device according to the second or the third aspect, the hybrid workvehicle further comprises: an assist/limit control unit that limits thefirst AC power based upon a level of the second DC power. If theengagement state detection unit detects the engaged state while theengine rotation rate is being controlled based upon the pedal operationquantity by the engine control unit upon detection of the travelingstate by the travel state detection unit and detection of thenon-engaged state by the engagement state detection unit, the secondpower conversion unit converts power representing a sum of the second DCpower and the first DC power, to the second AC power.

According to the fifth aspect of the present invention, in the hybridwork vehicle according to any one of the second through fourth aspects,if the lever operation quantity detection unit detects the leveroperation quantity equal to or greater than a predetermined value whilethe travel state detection unit detects the traveling state and theengagement state detection unit detects the non-engaged state, theengine control unit further raises the engine rotation rate relative toa target rotation rate based upon the pedal operation quantity.

According to the sixth aspect of the present invention, in the hybridwork vehicle according to any one of the second through fifth aspects,the hydraulic pump is a variable-capacity hydraulic pump a capacity ofwhich can be adjusted by altering the tilt. The hybrid work vehiclefurther comprises: a tilt control unit that increases/decreases the tiltof the hydraulic pump in correspondence to the lever operation quantity;and a work state decision-making unit that makes a decision as towhether or not the hybrid work vehicle is engaged in excavation work. Asthe work state decision-making unit decides that the hybrid work vehicleis engaged in the excavation work, the tilt control unit limits the tiltto a predetermined value.

According to the seventh aspect of the present invention, in the hybridwork vehicle according to the sixth aspect, the work device includes anarm linked to a body so as to be allowed to rotate up/down and an armcylinder that drives the arm; and the control lever outputs at least araise command and a lower command for the arm. The hybrid work vehiclefurther comprises: an arm angle detection unit that detects an angle ofthe arm; and a forward/reverse command unit that outputs a forwardcommand instructing the hybrid work vehicle to move forward and areverse command instructing the hybrid work vehicle to move backward.When the raise command is output by the control lever, the forwardcommand is output by the forward/reverse command unit and the arm angledetection unit detects that the angle is less than a predeterminedvalue, the work state decision-making unit decides that the hybrid workvehicle is engaged in excavation work.

According to the eighth aspect of the present invention, in the hybridwork vehicle according to any one of the second through seven aspects,the travel state detection unit includes the pedal operation quantitydetection unit, detects the traveling state if the pedal operationquantity detection unit detects the pedal operation quantity equal to orgreater than a first predetermined value and detects the non-travelingstate if the pedal operation quantity detection unit detects the pedaloperation quantity less than the first predetermined value.

According to the ninth aspect of the present invention, in the hybridwork vehicle according to any one of the second through seventh aspects,the travel state detection unit includes a vehicle speed sensor thatdetects a vehicle speed, detects the traveling state if the vehiclespeed sensor detects a vehicle speed equal to or greater than apredetermined speed and detects the non-traveling state if the vehiclespeed sensor detects a vehicle speed less than the predetermined speed.

According to the tenth aspect of the present invention, in the hybridwork vehicle according to any one of the second through ninth aspects,the engagement state detection unit includes the lever operationquantity detection unit, detects the engaged state if the leveroperation quantity detection unit detects a lever operation quantityequal to or greater than a second predetermined value and detects thenon-engaged state if the lever operation quantity detection unit detectsa lever operation quantity less than the second predetermined value.

According to the eleventh aspect of the present invention, in the hybridwork vehicle according to any one of the second through ninth aspect,the engagement state detection unit includes a pressure sensor thatdetects an output pressure of the hydraulic pump, detects the engagedstate if the pressure sensor detects a pressure equal to or greater thana predetermined pressure and detects the non-engaged state if thepressure sensor detects a pressure less than the predetermined pressure.

Advantageous Effect of the Invention

According to the present invention, a hybrid work vehicle that enablesefficient operation in correspondence to the work mode is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation of a wheel loader representing an example ofthe hybrid work vehicle according to the present invention.

FIG. 2 presents an example of a structure that may be adopted in thewheel loader in an embodiment of the present invention.

FIG. 3 is a diagram indicating the relationship between a leveroperation quantity representing the extent to which the arm lever isoperated and the pilot pressure.

FIG. 4 presents a motor required torque map (motor characteristics).

FIG. 5(a) presents an electric power generation quantity map and FIG.5(b) presents an engine control map used in conjunction with thetraveling drive device.

FIG. 6 presents an engine control map for the work device.

FIG. 7 presents an engine rotation rate correction map used forcombination work.

FIG. 8 presents a tilt control map used for work device solo operation.

FIG. 9(a) presents a tilt control map used for excavation work and FIG.9(b) presents a tilt control map used for combination operation.

FIG. 10 is a detailed diagram pertaining to the regulator.

FIG. 11 is a graph indicating a relationship that may be assumed for thetarget drive current and the pump tilt.

FIG. 12 illustrates V-shape loading, which is one of the methods thatmay be adopted when loading dirt or the like into a dump truck.

FIG. 13 illustrates excavation work performed by a wheel loader.

FIG. 14 illustrates transportation work and loading work performed by awheel loader.

FIG. 15 presents a flowchart of an example of operation processing thatmay be executed by the main controller.

FIG. 16 presents a flowchart of an example of operation processing thatmay be executed by the main controller in the excavation work mode.

FIG. 17 presents a flowchart of an example of operation processing thatmay be executed by the main controller in the traveling drive devicesolo operation mode.

FIG. 18 presents a flowchart of an example of operation processing thatmay be executed by the main controller in the combination operationshift mode.

FIG. 19 presents a flowchart of an example of operation processing thatmay be executed by the main controller in the combination operationmode, in which the traveling drive device and the work device areengaged in operation together.

FIG. 20 presents a flowchart of an example of operation processing thatmay be executed by the main controller in the work device solo operationmode.

FIG. 21 illustrates a method with which energy is distributed underassist control.

DESCRIPTION OF EMBODIMENTS First Embodiment

An embodiment of the hybrid work vehicle according to the presentinvention will be described in reference to drawings.

FIG. 1 is a side elevation of a wheel loader 100 representing an exampleof the hybrid work vehicle according to the present invention. The wheelloader 100 includes a front body 110 and a rear body 120. An arm 111, abucket 112, front wheels 113 and the like are disposed at the front body110. An operator's cab 121, an engine compartment 122, rear wheels 123and the like are disposed at the rear body 120.

The arm 111, linked to the front body 110 so as to be allowed to rotateup/down (articulate up/down) as it is driven by an arm cylinder 117. Thebucket 112, linked to the front end of the arm 111 so as to be allowedto rotate up/down (engage in a digging operation or a dumping operation)as it is driven by a bucket cylinder 115. The front body 110 and therear body 120 are connected with each other via a center pin 101 so asto articulate freely relative to each other. As a steering cylinder (notshown) extends/contracts, the front body 110 pivots to the left or tothe right relative to the rear body 120 and thus, the front body 110 issteered.

An arm angle sensor 54 (see FIG. 2), which detects the rotational angleof the arm 111 relative to the front body 110, is disposed at a rotatingportion of the arm 111. The arm angle sensor 54 may be, for instance, arotary potentiometer.

FIG. 2 presents an example of a structure that the wheel loader 110 mayadopt. The wheel loader 100 includes a main controller 20, and engine 1,an engine controller 21, a traveling dynamo electric device 100E, ahydraulic pump 10, a pump controller 60, a work device 100H and atraveling drive device 100D. The work device 100H may be, for instance,a hydraulic work device.

The work device 100H, which includes the arm 111 and the bucket 112shown in FIG. 1 and the arm cylinder 117 and the bucket cylinder 115shown in FIG. 2, is driven with pressure oil supplied from the hydraulicpump 10.

The hydraulic pump 10 is mechanically connected to the engine 1 andoutputs pressure oil as it is driven by the engine 1. The hydraulic pump10 is a variable capacity hydraulic pump, the capacity of which can beadjusted by varying the tilt angle of a swash plate 10 a to be describedlater. The tilt angle of the swash plate 10 a in the hydraulic pump 10may be otherwise referred to as the tilt of the hydraulic pump 10 or thepump tilt. Hydraulic operating fluid in a tank 90 is supplied via thehydraulic pump 10 to the arm cylinder 117 and the bucket cylinder 115via a control valve 11. The tilt angle can be adjusted with a regulator6. By altering the pump output capacity through adjustment of the tiltangle of the hydraulic pump 10, the output flow rate relative to therotation rate can be controlled.

As the pressure oil output from the hydraulic pump 10 is guided to thearm cylinder 117 and the bucket cylinder 115, which are hydraulic workactuators, via the control valve 11, the actuators are driven. Thecontrol valve 11 is controlled by a hydraulic signal or an electricsignal output from an operation device such as a control lever,installed in the operator's cab 121. The hydraulic operating fluidsupplied to the control valve 11 by the hydraulic pump 10 is distributedto the individual hydraulic actuators in correspondence to the operationof the operation device. Thus, the operator is able to control theextension/contraction of each hydraulic actuator simply by operating thecorresponding control lever.

Operation devices disposed within the operator's cab 121 include an armlever 57, which is a control lever operated to extend/contract the armcylinder 117, a bucket lever 58, which is a control lever operated toextend/contract the bucket cylinder 115, a steering wheel (not shown)operated to extend/contract the steering cylinder (not shown) and thelike. A forward/reverse changeover switch 51 operated to switch thetraveling direction of the wheel loader 100 to forward or reverse, anaccelerator pedal (not shown) and a brake pedal (not shown) areinstalled in the operator's cab 121.

The arm lever 57 is a control lever with which the arm 111 is operated,and a raise/lower command for the arm 111 is output from the arm lever57. The arm lever 57 is a hydraulic pilot-type operation lever. As FIG.3 indicates, a pilot pressure is output in correspondence to the leveroperation quantity (lever stroke) representing the extent to which thearm lever 57 is operated. As long as the lever operation quantity L isless than a predetermined value La, the pilot pressure p does notincrease. However, once the lever operation quantity L becomes equal tothe predetermined value La, the pilot pressure p increases to apredetermined value pa.

While the lever operation quantity L stays within a range between thevalue La and a value Lb, the pilot pressure p increases in proportion tothe lever operation quantity L. The bucket lever 58 is a hydraulicpilot-type operation lever similar to the arm lever 57, and a dig/dumpcommand for the bucket 112 is output from the bucket lever 58.

As the steering cylinder (not shown) extends/contracts in response to anoperation of the steering wheel performed by the operator, the steeringangle of the vehicle is adjusted and, as a result, the vehicle is causedto turn. As the operator operates the arm lever 57 and the bucket lever58 and the arm cylinder 117 and the bucket cylinder 115 are caused toextend/contract by the operator, the height of the arm 111 and theinclination of the bucket 112 are controlled by the operator and thus,the wheel loader can be engaged in excavation and loading work by theoperator.

As FIG. 2 indicates, the traveling dynamo-electric device 100E includesa motor/generator (M/G) 5, an M/G inverter 25, a traveling motor 4, atraveling inverter 24, a power storage element 7 and a converter 27. Thepower storage element 7 may be, for instance, a capacitor. The travelingdrive device 100D, which includes propeller shafts 40F and 40R,differential devices 41F and 41R, axles 42F and 42R, a pair of frontwheels 113 and a pair of rear wheels 123, is driven by the travelingmotor 4. The front wheel-side propeller shaft 40F and the rearwheel-side propeller shaft 40R are linked with each other via auniversal joint 45.

The pair of front wheels 113 are each connected to the front wheel-sideaxle 42F. The front wheel-side axle 42F is linked to the frontwheel-side propeller shaft 40F via the differential device 41F. The pairof rear wheels 123 are each connected to the rear wheel-side axle 42R.The rear wheel-side axle 42R is linked to the rear wheel side propellershaft 40R via the differential device 41R. The traveling motor 4 ismounted on the axis of the rear wheel-side propeller shaft 40R.

The motor/generator 5, linked to the output shaft of the engine 1,functions as a power generator and generates three-phase AC power, whendriven by the engine 1. This three-phase AC power is converted to DCpower via the M/G inverter 25 and the DC power is supplied to thetraveling inverter 24. When the charging rate of the power storageelement 7 is down to a predetermined value, the DC power resulting fromthe conversion by the M/G inverter 25 is also provided to the powerstorage element 7 via the converter 27. The power storage element 7 isthus charged.

The M/G inverter 25 and the traveling inverter 24 convert DC power to ACpower or AC power to DC power. The M/G inverter 25 and the travelinginverter 24 are connected to the power storage element 7 via theconverter 27. The converter 27 raises or lowers the charge/dischargevoltage of the power storage element 7.

The power storage element 7 is an electric double-layer capacitorcapable of storing power generated through a certain level of electricwork (e.g., work over several seconds at several tens of kW) anddischarging the stored charge with desired timing. The power storageelement 7 is charged with DC power resulting from conversion at thetraveling inverter 24 or the M/G inverter 25.

The DC power resulting from conversion at the M/G inverter 25 and/or DCpower output from the power storage element 7 is converted tothree-phase AC power by the traveling inverter 24. The traveling motor4, driven with the three-phase AC power resulting from the conversion bythe traveling inverter 24, generates rotational torque. The rotationaltorque generated at the traveling motor 4 is transmitted to the wheelsvia the traveling drive device 100D.

During a regenerative braking operation, the traveling motor 4 rotateswith a rotational torque transmitted from the wheels and, as a result,three-phase AC power is generated. The three-phase AC power generatedthrough the rotation of the traveling motor 4 is then converted to DCpower by the traveling inverter 24 and is supplied to the power storageelement 7 via the converter 27. The power storage element 7 is chargedwith the DC power resulting from the conversion by the travelinginverter 24.

The main controller 20 and the engine controller 21 each include anarithmetic processing device. The arithmetic processing devices eachinclude a CPU, a ROM and a RAM used as storage devices, other peripheralcircuits and the like. The main controller 20 executes control of theoverall system including the traveling system and the hydraulic worksystem in the wheel loader 100. The main controller 20 controls thevarious units so as to enable the entire system to perform at themaximum level.

Signals output from the forward/reverse changeover switch 51, anaccelerator pedal sensor 52, a vehicle speed sensor 53, the arm anglesensor 54, a pump pressure sensor 55, a pilot pressure sensor 56, anengine rotation rate sensor 50 and a traveling motor rotation ratesensor 59 are input to the main controller 20.

The forward/reverse changeover switch 51 outputs a forward commandsignal, carrying a command for the traveling drive device 100D to drivethe wheel loader 100 forward, and a reverse command signal, carrying acommand for the traveling drive device 100D to drive the wheel loader100 in the reverse direction, to the main controller 20. The acceleratorpedal sensor 52 detects the pedal operation quantity at the acceleratorpedal (not shown) and outputs an acceleration signal indicating thepedal operation quantity at the accelerator pedal, to the maincontroller 20. The vehicle speed sensor 53 detects the vehicle speed ofthe wheel loader 100 and outputs a vehicle speed signal indicating thevehicle speed of the wheel loader 100, to the main controller 20. Thearm angle sensor 54 detects the angle of the arm 111 and outputs anangle signal indicating the angle of the arm 111 to the main controller20.

The pump pressure sensor 55 detects the output pressure of the hydraulicpump 10 and outputs a pump pressure signal, indicating the outputpressure of the hydraulic pump 10, to the main controller 20. The pilotpressure sensor 56 detects a pilot pressure representing the leveroperation quantity of the arm lever 57 and outputs a lever signal,indicating the pilot pressure, to the main controller 20. The wording“the pilot pressure sensor 56 outputting the lever signal” may beotherwise simply described as “the arm lever 57 outputting a command”.The engine rotation rate sensor 50 detects the actual rotation rate ofthe engine 1 and outputs an actual rotation rate signal, indicating theactual rotation rate of the engine 1, to the main controller 20. Thetraveling motor rotation rate sensor 59 detects the rotation rate of thetraveling motor 4 and outputs a motor rotation rate signal, indicatingthe rotation rate of the traveling motor 4, to the main controller 20.

Based upon the lever operation quantity of the control lever and/or thepedal operation quantity of the accelerator pedal (not shown), the maincontroller 20 sets a target rotation rate for the engine 1 that isoptimal for the current work mode, and outputs a target rotation ratecommand, indicating the target rotation rate of the engine 1 having beenset for the engine 1, to the engine controller 21.

The main controller 20 controls the engine 1 and the inverters 24 and 25so that the traveling motor 4 outputs a required torque correspondingto, for instance, the pedal operation quantity of the accelerator pedal(not shown). The main controller 20 sets the target rotation rate of theengine 1 in order to have the motor/generator 5 generate the powerrequired by the traveling motor 4, and outputs the target rotation ratecommand, indicating the target rotation rate of the engine 1 having beenset, to the engine controller 21.

The main controller 20 determines through arithmetic operation a motorrequired torque, which is a torque required by the traveling motor 4during traveling operation. FIG. 4 presents a motor required torque map(motor characteristics). The motor required torque map indicates torquecurves pertaining to the traveling motor 4. The motor required torquemap is set so that the motor required torque Tr is in proportion to theacceleration signal and in reverse proportion to the rotation rate ofthe traveling motor 4. The motor required torque map is stored in astorage device within the main controller 20.

At the main controller 20, a relationship whereby the output of thetraveling motor 4 increases/decreases in correspondence to anincrease/decrease in the value indicated by the acceleration signalinput thereto from the accelerator pedal sensor 52, is set for theacceleration signal and the output of the traveling motor 4. The maincontroller 20 determines the torque curve corresponding to theacceleration signal input thereto and determines the motor requiredtorque Tr corresponding to the current rotation rate of the travelingmotor 4 by referencing the torque curve.

For instance, in response to a full operation of the accelerator pedal,the main controller 20 determines the 100% characteristics in FIG. 4 forthe maximum output of the traveling motor 4. Based upon the 100% maximumoutput characteristics and the current rotation rate of the travelingmotor 4, the main controller 20 determines the motor required torque Trcorresponding to the current rotation rate. Based upon this motorrequired torque Tr, the main controller 20 generates a motor drivesignal through a method of the known art and outputs the motor drivesignal thus generated to the traveling inverter 24.

The main controller 20 determines through arithmetic operation theelectric power generation quantity to be generated by themotor/generator 5. FIG. 5(a) presents an electric power generationquantity map. The electric power generation quantity map is set so thatthe electric power generation quantity Pe representing the electricpower to be generated is in proportion to the motor required torque Trover a motor required torque range within which Tr=Tr_min˜Tr_max. Theelectric power generation map is stored in a storage device within themain controller 20. The main controller 20 references this electricpower generation map and calculates the electric power generationquantity Pe based upon the motor required torque Tr having beencalculated. The main controller 20 outputs an engine drive controlsignal generated based upon a target engine rotation rate to bedescribed later, to the engine controller 21 so as to have themotor/generator 5 generate power in the electric power generationquantity Pe, having been calculated as described above, at. Togetherwith the engine drive control signal output to the engine controller 21,the main controller 20 also outputs a drive signal, which is used toconvert the three-phase AC power generated by the motor/generator 5 toDC power, to the M/G inverter 25. The electric power generation quantityPe and the motor required torque Tr do not need to have a directproportional relationship. They may instead assume, for instance, aproportional relationship represented by a quadratic curve or they mayassume a relationship whereby they increase in steps.

The main controller 20 determines through arithmetic operation thetarget rotation rate of the engine 1 based upon the electric powergeneration quantity Pe having been calculated. FIG. 5(b) presents anengine control map used in conjunction with the traveling drive device.The engine control map for the traveling drive device is set so that thetarget engine rotation rate Nt is in proportion to the electric powergeneration quantity Pe over an electric power generation quantity rangewithin which Pe=Pe_min˜Pe_max. The engine control map for the travelingdrive device is stored in a storage device within the main controller20. The main controller 20 references this engine control map for thetraveling drive device and calculates the target rotation rate Nt of theengine 1 based upon the electric power generation quantity Pe havingbeen calculated. The main controller 20 outputs a signal indicating thetarget rotation rate Nt to the engine controller 21. The target rotationrate Nt of the engine 1 and the electric power generation quantity Pe donot need to have a direct proportional relationship. They may insteadassume, for instance, a proportional relationship represented by aquadratic curve or they may assume a relationship whereby they increasein steps.

As described above, the engine rotation rate can be adjusted in responseto an operation at the arm lever 57 in the embodiment. When the workdevice 100H alone is driven, the main controller 20 calculates thetarget rotation rate of the engine 1 in correspondence to the leveroperation quantity L. FIG. 6 presents an engine control map for the workdevice. The engine control map for the work device is set so that thetarget engine rotation rate Nt is in proportion to the pilot pressure pover a pilot pressure range within which p=pa˜pb. The engine control mapfor the work device is stored in a storage device within the maincontroller 20. When the pilot pressure p is equal to pa, control isexecuted so as to hold the engine rotation rate at a minimum rotationrate Nt_min, whereas when the pilot pressure p is equal to pb, controlis executed so as to raise the engine rotation rate to a maximumrotation rate Nt_max. The main controller 20 references this enginecontrol map for the work device and calculates the target rotation rateNt of the engine 1 based upon the pilot pressure p indicated in thelever signal input thereto. The main controller 20 outputs a signalindicating the target engine rotation rate Nt to the engine controller21. The target engine rotation rate Nt and the pilot pressure p do notneed to have a direct proportional relationship. They may insteadassume, for instance, a proportional relationship represented by aquadratic curve or they may assume a relationship whereby they increasein steps.

The wheel loader 100 is engaged in combination work. For instance, itraises the arm 111 or performs excavation work while traveling. In sucha situation, the operator operates the control lever while stepping onthe accelerator pedal so as to engage both the traveling drive device100D and the work device 100H at the same time. In order to drive thetraveling drive device 100D and the work device 100H, the maincontroller 20 determines through arithmetic operation a target rotationrate Nt of the engine 1 based upon the pedal operation quantity of theaccelerator pedal and the lever operation quantity L of the controllever. An engine rotation rate correction map to be used for combinationwork is stored in a storage device at the main controller 20.

FIG. 7 presents the engine rotation rate correction map used forcombination work. The engine rotation rate correction map forcombination work is set so that a correction rotation rate ΔN is inproportion to the pilot pressure p over a pilot pressure range withinwhich p=p1˜pb. Namely, the engine rotation rate correction map forcombination work is set so that the engine rotation rate increases asthe pilot pressure p takes on greater values relative to a predeterminedvalue p1. When the traveling drive device 100D and the work device 100Hare engaged in combination operation, the main controller 20 calculatesa target engine rotation rate Ntc by adding a correction rotation rateΔN to the target engine rotation rate Nt (Ntc=Nt+ΔN), and outputs asignal indicating the corrected target engine rotation rate Ntc thusobtained to the engine controller 21. The pre-correction target enginerotation rate Nt is calculated based upon the electric power generationquantity Pe, by referencing the engine control map for the travelingdrive device presented in FIG. 5(b). A correction rotation rate ΔN iscalculated based upon the pilot pressure p, by referencing the enginerotation rate correction map for combination work presented in FIG. 7. Acorrection rotation rate ΔN and the pilot pressure p do not need to havea direct proportional relationship. They may instead assume, forinstance, a proportional relationship represented by a quadratic curveor they may assume a relationship whereby they increase in steps.

The engine controller 21 compares the actual rotation rate Na of theengine detected by the engine rotation rate sensor 50 with the targetengine rotation rate Nt or the corrected target engine rotation rate Ntcoutput by the main controller 20, and controls the fuel injection device(not shown) so as to adjust the actual engine rotation rate Na close tothe target engine rotation rate Nt or the corrected target enginerotation rate Ntc.

The main controller 20 executes charge/discharge control for the powerstorage element 7 so as to sustain the charging rate (i.e., SOC: stateof charge) of the power storage element 7 within a predetermined range.

Based upon the arm angle signal input thereto and an arm angle thresholdvalue stored in a storage device, the main controller 20 makes adecision as to whether or not the angle of the arm 111 is less than thethreshold value. Deciding whether or not the angle of the arm 111 isless than the threshold value is equivalent to making a decision as towhether or not the height of the arm 111 is less than a predeterminedheight measured from the travelling surface. The arm angle thresholdvalue may represent a rotational angle assumed by the arm 111 when thearm 111 is, for instance, approximately 300 mm above the travellingsurface. As an alternative, a decision as to whether or not the heightof the arm 111, calculated based upon geometric information pertainingto the wheel loader 100, stored in a storage device at the maincontroller 20, and the angle detected by the arm angle sensor 54, isless than a predetermined value may be made.

In correspondence to the current work mode, the main controller 20selects one of a plurality of tilt control maps set in advance, anddetermines through arithmetic operation a target tilt q of the hydraulicpump 10 based upon the pilot pressure p by referencing the selected tiltcontrol map. FIG. 8 presents a tilt control map to be used for workdevice solo operation. The tilt control map for work device solooperation is set so that the tilt q is in proportion to the pilotpressure p over a range within which the pilot pressure p indicated inthe lever signal is between pa and pb. The tilt control map for workdevice solo operation is stored in a storage device within the maincontroller 20.

A pilot pressure p that is equal to pa corresponds to the minimum leveroperation quantity La set by taking into consideration the dead zone ofthe control lever. When the pilot pressure p is equal to or greater thanthe predetermined value pa, the control valve 11 is switched. A pilotpressure p that is equal to pb corresponds to the maximum leveroperation quantity Lb of the control lever (see FIG. 3).

FIG. 9(a) presents a tilt control map for excavation work. The term“excavation work” is used to refer to work of the wheel loader performedby scooping up dirt when driving the wheel loader 100 into dirt anddigging dirt with the bucket 112. The tilt control map for excavationwork is set so that the tilt q is in proportion to the pilot pressure pover a range within which the pilot pressure p, indicated by the leversignal, is between pa and p2. The tilt control map for excavation workis stored in a storage device within the main controller 20.

As FIG. 9(a) indicates, when the control lever is operated with a leverstroke with the pilot pressure p exceeding the predetermined value p2,the tilt q takes on a constant value q_L. In this situation,restrictions are imposed on the input torque of the hydraulic pump 10,since priority must be given to assuring tractive force when the bucket112 is driven into dirt to scoop up dirt by moving the wheel loader 100forward.

FIG. 9(b) presents a tilt control map used for combination operation.The term “combination operation” is used in this context to refer to acombination operation which does not include the excavation workdescribed above, e.g., an operation of the wheel loader 100 to travelraising the arm 111 of the wheel loader 100. The tilt control map forcombination operation is set so that the tilt q is in proportion to thepilot pressure p over a range within which the pilot pressure p,indicated by the lever signal, is between p2 and pb. The tilt controlmap for combination operation is stored in a storage device within themain controller 20.

The signal (lever signal) indicating the pilot pressure is output by thepump controller 60 as a target drive current to the regulator 6. Theregulator 6 adjusts the tilt of the hydraulic pump 10. The tilt of thehydraulic pump 10 corresponds to the output volume. The following is adetailed description of the tilt control achieved by the pump controller60 and the regulator 6, given in reference to the particular instance oftilt control executed based upon the tilt control map for work devicesolo operation presented in FIG. 8.

FIG. 10 shows the regulator 6 in detail. The regulator 6 controls thetilt angle of the hydraulic pump 10 based upon the target drive currenti0 output by the pump controller 60 so that the tilt angle is adjustedto match a target pump tilt angle corresponding to the target drivecurrent i0. The regulator 6 includes an electromagnetic proportionalpressure-reducing valve 64, a servo valve 61 and a servo piston 62. Asthe target drive current i0 output by the pump controller 60 is input tothe electromagnetic proportional pressure-reducing valve 64, theelectromagnetic proportional pressure-reducing valve 64 outputs acommand pressure that is in proportion to the target drive current i0 tothe servo valve 61. The servo valve 61 is engaged in operation with thecommand pressure so as to control the position of the servo piston 62.The servo piston 62 drives the swash plate 10 a at the hydraulic pump 10to control the tilt angle thereof.

The output pressure of the hydraulic pump 10 is guided to an input portof the servo valve 61 via a check valve 63 and is also constantly inaction on a small diameter chamber 62 a of the servo piston 62 via apassage 65. As the output pressure from a pilot pump 66 is guided to aninput port of the electromagnetic proportional pressure-reducing valve64 and the electromagnetic proportional pressure-reducing valve 64 isthus engaged in operation, the output pressure, having been guided tothe electromagnetic proportional pressure-reducing valve 64, is reducedand the resulting lowered pressure is used as a command pressure. Thiscommand pressure is communicated through a passage 67 and acts on apilot piston 61 a of the servo valve 61. If the output pressure from thehydraulic pump 10 is lower than the output pressure from the pilot pump66, the output pressure from the pilot pump 66, now to be used as aservo assistance pressure, is guided to the input port of the servovalve 61 via a check valve 69.

FIG. 11 indicates a relationship that may be assumed for the targetdrive current i0 provided to the electromagnetic proportionalpressure-reducing valve 64 and the tilt angle assumed by the swash plate10 a in the hydraulic pump 10. The relationship indicated in FIG. 11corresponds to the tilt control map presented in FIG. 8.

As long as the target drive current i0 is equal to or less than a valueR1, the electromagnetic proportional pressure-reducing valve 64 remainsdisengaged and the command pressure output from the electromagneticproportional pressure-reducing valve 64 is 0. In this state, a spool 61b of the servo valve 61 is pushed to the left in FIG. 10, by a spring 61c. Under these circumstances, the output pressure from the hydraulicpump 10 (or the output pressure from the pilot pump 66) travels throughthe check valve 63, a sleeve 61 d and the spool 61 b and acts on a largediameter chamber 62 b of the servo piston 62. While the output pressurefrom the hydraulic pump 10 is also acting at the small diameter chamber62 a in the servo piston 62 via the passage 65, the servo piston 62 isdisplaced to the right in FIG. 10, due to the area difference.

As the servo piston 62 moves to the right in FIG. 10, a feedback lever71 rotates on a fulcrum formed at a pin 72 along the counterclockwisedirection in FIG. 10. Since the front end of the feedback lever 71 islinked with the sleeve 61 d at the pin 73, the sleeve 61 d becomesdisplaced to the left in FIG. 10. The displacement of the servo piston62 continues until communication between an opening portion at thesleeve 61 d and a notch formed at the spool 61 b becomes cut off, andonce they are completely cut off from each other the servo piston 62comes to a stop.

Through this sequence, the tilt angle of the hydraulic pump 10 isadjusted to achieve a minimum value qmin, resulting in the minimum flowrate for the output from the hydraulic pump 10.

As the target drive current i0 becomes greater than the value R1 in thegraph presented in FIG. 11 and the electromagnetic proportionalpressure-reducing valve 64 becomes engaged, a command pressure, whichcorresponds to the extent to which the electromagnetic proportionalpressure-reducing valve 64 is operated, is applied to the pilot piston61 a of the servo valve 61 through the passage 67. In this situation,the spool 61 b moves to the right in FIG. 10, to a position at which abalance with the force imparted from the spring 61 c is achieved. Thedisplacement of the spool 61 b causes the large diameter chamber 62 b ofthe servo piston 62 to become connected with the tank 90 via a passageformed inside the school 61 b. Since the output pressure from thehydraulic pump 10 (or the output pressure from the pilot pump 66),provided via the passage 65, acts at all times on the small diameterchamber 62 a of the servo piston 62, the servo piston 62 moves to theleft in FIG. 10, and the hydraulic operating fluid in the large diameterchamber 62 b is allowed to travel back into the tank 90.

As the servo piston 62 moves to the left in FIG. 10, the feedback lever71 rotates on the fulcrum formed at the pin 72 along the clockwisedirection in FIG. 10, and the sleeve 61 d of the servo valve 61 becomesdisplaced to the right in FIG. 10. The displacement of the servo piston62 continues until communication between the opening portion at thesleeve 61 d and the notch formed at the spool 61 b becomes cut off, andonce they are completely cut off from each other the servo piston 62comes to a stop.

Through this sequence, the tilt angle at the hydraulic pump 10 isincreased, which results in an increase in the flow rate of the outputfrom the hydraulic pump 10. The flow rate of the output from thehydraulic pump 10 increases in proportion to the extent to which thecommand pressure rises, i.e., the extent to which the target drivecurrent i0 increases.

As the target drive current i0 decreases and the command pressure outputfrom the electromagnetic proportional pressure-reducing valve 64 becomeslower, the spool 61 b of the servo valve 61 is caused to move backtoward the left in FIG. 10, until it reaches the position at which abalance with the force imparted from the spring 61 c is achieved. Theoutput pressure from the hydraulic pump 10 (or the output pressure fromthe pilot pump 66), provided through the sleeve 61 d and the spool 61 bof the servo valve 61, acts on the large diameter chamber 62 b of theservo piston 62, and the servo piston 62 is caused to move to the rightin FIG. 10, due to the difference between the area of the large diameterchamber 62 b and the area of the small diameter chamber 62 a.

As the servo piston 62 moves to the right in FIG. 10, the feedback lever71 rotates on the fulcrum formed at the pin 72 along thecounterclockwise direction in FIG. 10, and the sleeve 61 d of the servovalve 61 becomes displaced to the left in FIG. 10. The displacement ofthe servo piston 62 continues until communication between an openingportion at the sleeve 61 d and a notch formed at the spool 61 b becomescut off, and once the communication is completely cut off, the servopiston 62 comes to a stop.

Through this sequence, the tilt angle at the hydraulic pump 10 isdecreased, which results in a decrease in the flow rate of the outputfrom the hydraulic pump 10. The flow rate of the output from thehydraulic pump 10 decreases in proportion to the extent to which thecommand pressure falls, i.e., the extent to which the target drivecurrent i0 decreases.

FIG. 12 illustrates how a V-shape loading operation, representing amethod that may be adopted when loading dirt or the like into a dumptruck, is performed. In the V-shape loading operation, the wheel loader100 is caused to advance forward, as indicated by an arrow a in order toscoop up dirt or the like.

The operator of the wheel loader 100 engaged in dirt scooping work,i.e., in the excavation work described earlier, normally drives thebucket 112 into a mound 130 of dirt or the like, as illustrated in FIG.13, and manipulates the bucket 112 before raising the arm 111, orsimultaneously manipulates the bucket 112 and the arm 111 before raisingthe arm 111 alone.

Once the particular excavation is completed, the wheel loader 100 backsaway, as indicated by an arrow b in FIG. 12. In this embodiment, theparticular excavation is judged to be completed when the operator hasswitched to “reverse” at the forward/reverse changeover switch 51. Then,as indicated by an arrow c, the wheel loader 100 advances forwardtowards a dump truck and stops in front of the dump truck.

As the wheel loader 100 travels forward toward the dump truck, the arm111 is raised, as illustrated in FIG. 14. If the bucket 112 has alreadybeen moved up to the loading height when the wheel loader 100 comes to astop before the dump truck, the dirt or the like having been scoopedinto the bucket 112 can be immediately emptied into the dump truck. Forthis reason, it is desirable that an optimal speed with which the arm111 is raised be assured while the wheel loader 100 travels forwardtoward the dump truck. Accordingly, the combination operation describedearlier is performed in this situation.

The embodiment allows the operator to adjust the load distributionbetween the traveling drive device 100D and the work device 100H. Thismeans that the operator may press the accelerator pedal only halfwaydown (half acceleration) and keep the arm lever 57 in an electromagnetichold (detent lock) at a raise position, so as to give higher priority tothe hydraulic work system over the traveling system. Through thesemeasures, the work efficiency can be improved, since the arm 111 can beraised with the optimal speed while the wheel loader 100 travels forwardso as to ensure that when the wheel loader 100 reaches a point in frontof the dump truck, the bucket 112 will have already been raised to theloading height.

Once the particular load is completed, the wheel loader 100 backs out tothe initial position, as indicated by an arrow d in FIG. 12. Thedescription provided above covers the basic operational flow of theloading method using the V-shape loading.

FIG. 15 through FIG. 20 each present a flowchart of an example ofoperation processing that may be executed by the main controller 20. Theprocessing executed as shown in the flowcharts is initially started upas, for instance, an engine key switch (not shown) is turned on. It isto be noted that the following description will only relate to theaspect of the hydraulic work performed when the arm lever 57 alone hasbeen operated and that no illustration or description of the SOC controlwill be provided.

As shown in FIG. 15, in step S1, the main controller 20 reads thesignals provided from the various sensors, levers and switches. The maincontroller 20 determines the current work mode of the wheel loader 100based upon the detection values having been read, and controls the driveof various units such as the engine 1, the traveling motor 4 and/or thehydraulic pump 10 in correspondence to the work mode.

The main controller 20 makes a decision as to which one of the followingstates is the current state of the wheel loader 100: a state in whichthe work device 100H alone is being driven, a state in which thetraveling drive device 100D alone is being driven, and a combinationoperation state in which the work device 100H and the traveling drivedevice 100D are both being driven. The main controller 20 in theembodiment makes a separate decision with regard to an excavation workstate, which is also a combination operation state.

In step S2, the main controller 20 makes a decision as to whether or notexcavation work is underway. Namely, the main controller 20 makes adecision as to whether or not the following conditions are allsatisfied: the arm lever 57 has been operated to the electromagnetichold position “raise” resulting in a raise command for the arm 111output from the arm lever 57, the forward/reverse changeover switch 51has been operated to the forward position resulting in a forward commandfor the traveling drive device 100D output from the forward/reversechangeover switch 51, and the signal provided by the arm angle sensor 54indicates an angle θ of the arm 111 less than a predetermined value θ1.When the angle θ of the arm 111 is less than the predetermined value θ1,the height h of the arm 111 is less than a predetermined value h1. Uponmaking an affirmative decision in step S2 in FIG. 1, the main controller20 determines that the wheel loader 100 is currently in the excavationwork state and the processing proceeds to step S3 to enter an excavationwork mode. If, on the other hand, a negative decision is made in stepS2, the processing proceeds to step S4.

In step S4, the main controller 20 makes a decision based upon theacceleration signal provided from the accelerator pedal sensor 52 as towhether or not the accelerator pedal has been operated to be stepped on.If a pedal operation quantity equal to or greater than a predeterminedvalue is detected via the accelerator pedal sensor 52, the maincontroller 20 decides in step S4 that a pedal operation has beenperformed, but if a pedal operation quantity less than the predeterminedvalue is detected via the accelerator pedal sensor 52, the maincontroller 20 decides in step S4 that no pedal operation has beenperformed.

Upon making an affirmative decision in step S4, the main controller 20determines that the wheel loader 100 is currently in a traveling state,in which the traveling drive device 100D is being driven. In this case,the processing proceeds to step S5, in which the main controller 20makes a decision based upon the pilot pressure signal provided from thepilot pressure sensor 56 as to whether or not the pilot pressure p isless than the predetermined value p1. Upon making an affirmativedecision in step S5, the main controller 20 determines that the wheelloader 100 is currently in a work device non-engaged state, in which thework device 100H is not being driven. In this case, the processingproceeds to step S6 to enter a traveling drive device solo operationmode. Upon making a negative decision in step S5, the main controller 20determines that the wheel loader 100 is currently in both the travelingstate, in which the traveling drive device 100D is being driven, and ina work device engaged state, in which the work device 100H is beingdriven. The processing proceeds to step S7 to enter a combinationoperation mode, in which the traveling drive device 100D and the workdevice 100H are both engaged in operation.

Upon making a negative decision in step S4, the main controller 20determines that the wheel loader 100 is currently in a non-travelingstate in which the traveling drive device 100D is not being driven. Inthis case, the processing proceeds to step S8, in which the maincontroller 20 makes a decision based upon the detection signal providedfrom the pilot pressure sensor 56 as to whether or not the pilotpressure p is equal to or greater than the predetermined value p1. Uponmaking an affirmative decision in step S8, the main controller 20determines that the wheel loader 100 is currently in a work deviceengaged state, in which the work device 100H is being driven. Theprocessing proceeds to step S9 to enter a work device solo operationmode.

In reference to FIG. 16, the flow of the operation processing executedby the main controller 20 in the excavation work mode will be explained.As FIG. 16 shows, in step S301, to which the processing proceeds afterentering the excavation work mode, the main controller 20 reads the tiltcontrol map to be used for excavation work (see FIG. 9(a)). In stepS303, the main controller 20 reads the engine control map to be used inconjunction with the traveling drive device (see FIG. 5(b), the electricpower generation map (see FIG. 5(a)) and the motor required torque map(see FIG. 4), before the processing proceeds to step S305. In step S305,the main controller 20 reads the engine rotation rate correction map forcombination work (see FIG. 7), and then the processing proceeds to stepS306. In step S306, the main controller 20 reads the accelerationsignal, the lever signal and the motor rotation rate signal, before theprocessing proceeds to step S311.

In step S311, the main controller 20 calculates, based upon theacceleration signal and the motor rotation rate signal having been read,a motor required torque Tr by referencing the motor required torque map(see FIG. 4), and then the processing proceeds to step S316. In stepS316, the main controller 20 calculates the electric power generationquantity Pe based upon the motor required torque Tr by referencing theelectric power generation map (see FIG. 5(a)) and then the processingproceeds to step S321. In step S321, the main controller 20 calculates,based upon the electric power generation quantity Pe, the targetrotation rate Nt of the engine 1 by referencing the engine control mapfor the traveling drive device (see FIG. 5(b)), and then the processingproceeds to step S326.

In step S326, the main controller 20 calculates, based upon the leversignal, a correction rotation rate ΔN by referencing the engine rotationrate correction map for combination work (see FIG. 7). The maincontroller 20 then calculates a corrected target rotation rate Ntc forthe engine 1 by adding a correction rotation rate ΔN calculated asdescribed above to the target rotation rate Nt initially of the engine1, and outputs the corrected target rotation rate Ntc of the engine 1 tothe engine controller 21. The engine controller 21 then compares theactual rotation rate Na of the engine 1 with the corrected targetrotation rate Ntc and controls the fuel injection device so as to adjustthe actual rotation rate Na of the engine 1 toward the corrected targetrotation rate Ntc.

In step S331, the main controller 20 outputs a drive signal to be usedto convert AC power in the electric power generation quantity Pe havingbeen calculated in step S316, to DC power, to the M/G inverter 25. Thethree-phase AC power generated by the motor/generator 5 is converted toDC power by the M/G inverter 25, and DC power in the electric powergeneration quantity Pe is provided to the traveling inverter 24.

In step S336, the main controller 20 outputs a drive signal to be usedto convert the DC power provided from the M/G inverter 25 to three-phaseAC power, to the traveling inverter 24. In response, the travelinginverter 24 converts the DC power resulting from the M/G inverter 25converting the three-phase AC signal provided from the motor/generator 5into DC, to three-phase AC power. The three-phase AC power resultingfrom the conversion by the traveling inverter 24 is then provided to thetraveling motor 4. As the three-phase AC power supplied to the travelingmotor 4 rotationally drives the traveling motor 4, a rotational torquecorresponding to the motor required torque Tr having been calculated instep S311 is generated, and the traveling drive device 100D is drivenwith this rotational torque.

In step S341, the main controller 20 outputs, based upon the leversignal, a signal to be used to control the electromagnetic proportionalpressure-reducing valve 64 in the regulator 6 by referencing the tiltcontrol map for excavation work (see FIG. 9(a)). During excavation work,the tilt of the hydraulic pump 10 is limited so that the extent of tiltdoes not exceed the predetermined value q_L. The predetermined value q_Lmay be, for instance, approximately 30% of a maximum tilt q_max.

In step S346, decision-making processing similar to that executed instep S2 described earlier is executed. Namely, the main controller 20makes a decision as to whether or not the following conditions are allsatisfied: the arm lever 57 has been operated to the electromagnetichold position “raise”, the forward/reverse changeover switch 51 has beenoperated to the forward position and the height h of the arm 111 isdetected to be less than the predetermined value h1.

If an affirmative decision is made in step S346, the processing returnsto step S306 to continue the processing in the excavation work mode,whereas if a negative decision is made in step S346, the processingexits the excavation work mode and the processing returns to step S1.

In the excavation work mode described above, the tilt of the hydraulicpump 10 is controlled based upon the tilt control map for excavationwork (see FIG. 9(a)) and the drive of the traveling motor 4 iscontrolled based upon the pedal operation quantity of the acceleratorpedal. In addition, the engine rotation rate is controlled based uponthe pedal operation quantity of the accelerator pedal and the leveroperation quantity of the arm lever 57. Since the arm lever 57 has beenoperated to the “raise” position and the operator is pressing on theaccelerator pedal, the engine rotation rate increases. However, the tiltof the hydraulic pump 10 is restricted in the lever operation quantityrange equal to and above a predetermined value (the pilot pressure p2),and thus, the maximum output is kept down. In other words, higherpriority is given to the traveling load over the work load, i.e., higherpriority is given to assurance of good tractive force, during theexcavation work (see FIG. 13).

If the arm 111 of a work vehicle equipped with a torque converter of therelated art is raised too fast during excavation work, the bucket 112will not bite into the excavation target, i.e., the dirt mound, to afull extent, and for this reason, the volume of material excavated maynot be optimal. If, on the other hand, the arm 111 is raised too slowlyduring excavation work, the bucket 112 may push too far into the mound,resulting in slippage (spinning) of the front wheels 113. In contrast,excavation work is performed in the embodiment with the arm raisingspeed regulated by controlling the pump tilt during excavation work, asindicated in FIG. 9(a), while at the same time generating a high levelof tractive force by outputting the required torque corresponding to theaccelerator pedal operation quantity via the traveling motor 4. As aresult, the excavation work can be performed with a high level ofefficiency.

In reference to FIG. 17, the flow of the operation processing executedby the main controller 20 in the traveling drive device solo operationmode will be explained. As FIG. 17 shows, in step S503, to which theprocessing proceeds after entering the traveling drive device solooperation mode, the main controller 20 reads the engine control map tobe used in conjunction with the traveling drive device (see FIG. 5(b)),the electric power generation quantity map (see FIG. 5(a)) and the motorrequired torque map (see FIG. 4), before the processing proceeds to stepS506. In step S506, the main controller 20 reads the accelerationsignal, the lever signal and the motor rotation rate signal, before theprocessing proceeds to step S511.

In step S511, the main controller 20 calculates, based upon theacceleration signal and the motor rotation rate signal having been read,a motor required torque Tr by referencing the motor required torque map(see FIG. 4), and then the processing proceeds to step S516. In stepS516, the main controller 20 calculates based upon the motor requiredtorque Tr, the electric power generation quantity Pe by referencing theelectric power generation quantity map (see FIG. 5(a)) and then theprocessing proceeds to step S521.

In step S521, the main controller 20 calculates, based upon the electricpower generation quantity Pe, the target rotation rate Nt of the engine1 by referencing the engine control map for the traveling drive device(see FIG. 5(b)), and outputs the target rotation rate Nt thus calculatedof the engine 1 to the engine controller 21. The engine controller 21compares the actual rotation rate Na of the engine 1 with the targetrotation rate Nt and controls the fuel injection device so as to adjustthe actual rotation rate Na of the engine 1 toward the target rotationrate Nt.

In step S531, the main controller 20 outputs a drive signal, to be usedto convert AC power in the electric power generation quantity Pe havingbeen calculated in step S516 to DC power, to the M/G inverter 25. Thethree-phase AC power generated by the motor/generator 5 is converted toDC power by the M/G inverter 25, and the DC power in the electric powergeneration quantity Pe is provided to the traveling inverter 24.

In step S536, the main controller 20 outputs a drive signal, to be usedto convert the DC power provided from the M/G inverter 25 to three-phaseAC power, to the traveling inverter 24. In response, the travelinginverter 24 converts the DC power resulting from the DC conversion ofthe three-phase AC signal provided from the motor/generator 5 at the M/Ginverter 25, to three-phase AC power. The three-phase AC power resultingfrom the conversion by the traveling inverter 24 is supplied to thetraveling motor 4. As the three-phase AC power supplied to the travelingmotor 4 rotationally drives the traveling motor 4, a rotational torquecorresponding to the motor required torque Tr having been calculated instep S511 is generated, and the traveling drive device 100D is drivenwith this rotational torque.

In step S541, the main controller 20 makes a decision as to whether ornot the pilot pressure p is equal to or greater than the predeterminedvalue p1. Upon making an affirmative decision in step S541, the maincontroller 20 determines that the wheel loader 100 is currently in thework device engaged state in which the work device 100H is being driven.In this case, the processing proceeds to step S550 to enter thecombination operation shift mode. If, on the other hand, a negativedecision is made in step S541, the processing proceeds to step S546.

In step S546, the main controller 20 makes a decision, based upon theacceleration signal provided by the accelerator pedal sensor 52, as towhether or not the accelerator pedal has been operated with a pedaloperation quantity equal to or greater than a predetermined value. Ifthe accelerator pedal sensor 52 has detected a pedal operation quantityequal to or greater than the predetermined value and an affirmativedecision is made in step S546 as a result, the processing returns tostep S506 to continuously hold the traveling drive device solo operationmode. If, on the other hand, the pedal operation quantity detected bythe accelerator pedal sensor 52 is less than the predetermined value anda negative decision is made in step S546 as a result, the processingexits the traveling drive device solo operation mode and returns to stepS1.

It is to be noted that although not shown, as the charging rate of thepower storage element 7 becomes lower while the wheel loader travels,the main controller 20 and the engine controller 21 increase the enginerotation rate and the resulting excess energy is used to charge thepower storage element 7.

As described above, in the operation mode that is set in the wheelloader 100 when the work device 100H is not being driven and is thus inthe non-engaged state while a traveling state, in which the travelingdrive device 100D is being driven, is also in effect, i.e., when thewheel loader 100 is in the traveling drive device solo operation mode,the rotation rates of the traveling motor 4 and the engine 1 arecontrolled based upon the pedal operation quantity detected by theaccelerator pedal sensor 52. For instance, while the wheel loader 100 isapproaching the mound 130, as indicated by the arrow a in FIG. 12, thework device 100H is not engaged and accordingly, the engine rotationrate is controlled based upon the electric power generation quantity Peneeded to rotate the traveling motor 4.

In reference to FIG. 18, the flow of the operation processing executedby the main controller 20 in the combination operation shift mode willbe explained. In a work vehicle equipped with a torque converter knownin the related art, the engine is controlled at a rotation rate set inadvance to a high value in order to ensure that the engine does notstall under a work load applied while the work vehicle is traveling. Incontrast, a stall of the engine 1 is prevented in the wheel loader 100achieved in the embodiment through the processing executed in thecombination operation shift mode, and thus, the need to allow for amargin in the engine rotation rate in the traveling drive device solooperation mode is eliminated. In other words, the embodiment enablesefficient rotation rate control for the engine 1, which makes itpossible to reduce fuel consumption, exhaust and noise.

As shown in FIG. 18, as the processing in the combination operationshift mode starts, the main controller 20 reads the tilt control map forcombination operation (see FIG. 9(b)) in step S551, and then theprocessing proceeds to step S553.

In step S553, the main controller 20 reads the engine rotation ratecorrection map for combination work (see FIG. 7), before the processingproceeds to step S555. In step S555, the main controller 20 reads theacceleration signal, the lever signal and the motor rotation ratesignal, before the processing proceeds to step S557. In step S557, themain controller 20 calculates, based upon the acceleration signal andthe motor rotation rate signal having been read, a motor required torqueTr by referencing the motor required torque map (see FIG. 4), and thenthe processing proceeds to step S559.

In step S559, the main controller 20 outputs a traveling motor assistcommand to the converter 27 and the traveling inverter 24. The converter27 boosts the DC power at the power storage element 7 and adds theboosted power to the DC power provided from the M/G inverter 25. Thecombined DC power is converted to three-phase AC power by the travelinginverter 24 and the three-phase AC power resulting from the conversionis supplied to the traveling motor 4, where it is used to rotationallydrive the traveling motor 4. As the traveling motor 4 is rotationallydriven, the traveling drive device 100D is driven.

In step S561, the main controller 20 turns on an electric powergeneration quantity limiting flag and the processing then proceeds tostep S566. In step S566, the main controller 20 calculates the electricpower generation quantity Pe based upon the motor required torque Tr byreferencing the electric power generation quantity map (see FIG. 5(a)),before the processing proceeds to step S571. In step S571, the maincontroller 20 calculates the target rotation rate Nt of the engine 1based upon the electric power generation quantity Pe by referencing theengine control map for the traveling drive device (see FIG. 5(b)), andthe processing then proceeds to step S573.

In step S573, the main controller 20 sets, as a corrected electric powergeneration quantity Pec=Pe−ΔPe, the difference obtained by subtractingan assist electric power generation quantity ΔPe, attributed to thepower storage element 7, from the electric power generation quantity Pecalculated in step S566, and then the processing proceeds to step S576.The main controller 20 calculates the required engine output based uponthe pump tilt angle and the pump output pressure, and designates anelectric power generation quantity equivalent to this output as theassist electric power generation quantity ΔPe.

In step S576, the main controller 20 calculates a correction rotationrate ΔN based upon the lever signal by referencing the engine rotationrate correction map for combination work (see FIG. 7). The maincontroller 20 calculates a corrected target rotation rate Ntc of theengine 1 by adding a correction rotation rate ΔN having been calculatedas described above to the target rotation rate Nt of the engine 1, andoutputs the corrected target engine rotation rate Ntc of the engine 1 tothe engine controller 21. The engine controller 21 compares the actualrotation rate Na of the engine 1 with the corrected target rotation rateNtc, and controls the fuel injection device so as to adjust the actualrotation rate Na of the engine 1 toward the corrected target rotationrates Ntc.

In step S581, a drive signal to be used to obtain DC power in theelectric power generation quantity Pec=Pe−ΔPe calculated in step S566,is output to the M/G inverter 25. Namely, the output of the AC powergenerated by the motor/generator 5 is limited by an extent equivalent tothe assist electric power generation quantity ΔPe described earlier. Thethree-phase AC power (Pec=Pe−ΔPe) generated by the motor/generator 5 isconverted to DC power (Pec) by the M/G inverter 25. DC power (Pe),representing the sum of this DC power (Pec) and the DC power (ΔPe)provided from the power storage element 7 in the assist electric powergeneration quantity combined together, is supplied to the travelinginverter 24.

In step S586, the main controller 20 outputs a drive signal to be usedto convert the DC power to three-phase AC power to the travelinginverter 24. The traveling inverter 24 converts the DC power (Pe)supplied to the traveling inverter 24 to three-phase AC power andsupplies the three-phase AC power resulting from the conversion to thetraveling motor 4. As the traveling motor 4 is rotationally driven withthe three-phase AC power supplied thereto, a rotational torquecorresponding to the motor required torque Tr calculated in step S557 isgenerated, and the traveling drive device 100D is driven with thisrotational torque.

In step S591, the main controller 20 outputs a signal to be used tocontrol the electromagnetic proportional pressure-reducing valve 64 inthe regulator 6, which is generated, based upon the lever signal, byreferencing the tilt control map for combination operation (see FIG.9(b)). In the combination operation shift mode, the tilt of thehydraulic pump 10 increases over a range in which the pilot pressure pis equal to or greater than the predetermined value p2. This means thatwhen the wheel loader shifts from the traveling drive device solooperation mode into the combination operation mode, the tilt starts toincrease relative to the lever operation quantity with timing retardedcompared to the timing with which the tilt starts to increase during theexcavation work described earlier and during the work device solooperation, which will be described in detail later. Since the work load,which does not apply in the traveling drive device solo operation mode,will not come into effect until the pilot pressure p becomes equal tothe predetermined value p2, even after the pilot pressure p reaches thepredetermined value p1 in response to a lever operation and the wheelloader shifts into the combination operation mode, an engine stall isprevented.

In step S596, the main controller 20 makes a decision as to whether ornot the actual rotation rate Na of the engine has been adjusted to avalue close to the target rotation rate Nt. The main controller 20determines that the actual rotation rate Na≈target rotation rate Ntc ifthe difference between the actual rotation rate Na of the engine 1 andthe corrected target rotation rate Ntc is less than a predeterminedvalue. Upon making an affirmative decision in step S596, the processingexits the combination operation shift mode and returns to step S1. Whenexiting the combination operation shift mode, the main controller 20resets the electric power generation quantity limiting flag. If, on theother hand, a negative decision is made in step S596, the processingreturns to step S555 and the processing in the combination operationshift mode is continuously executed.

As described above, in the combination operation shift mode, the tilt ofthe hydraulic pump 10 is controlled based upon the tilt control map forcombination operation (see FIG. 9(b)) and the drive of the travelingmotor 4 is controlled based upon the pedal operation quantity of theaccelerator pedal. In addition, the engine rotation rate is controlledbased upon the pedal operation quantity of the accelerator pedal and thelever operation quantity of the arm lever 57.

While the engine rotation rate is corrected so that it increases incorrespondence to the lever signal generated in response to an operationall of the arm lever 57 performed by the operator in the travelingstate, the actual rotation rate Na of the engine 1 does not immediatelyincrease to the corrected target rotation rate Ntc. In the embodiment,the traveling motor 4 is transiently assisted with power provided fromthe power storage element 7 so as to limit the power generation load byreducing the electric power generation quantity by an extentcorresponding to the assist. As a result, an unintended stoppage of theengine 1, i.e., an engine stall, can be prevented.

An instance in which a work load is added while power Pge=50 kW issupplied from the M/G inverter 25 to the traveling inverter 24 and powerPce=0 kW is supplied from the converter 27 to the traveling inverter 24(the traveling drive device solo operation mode), as illustrated in theconceptual diagram presented in FIG. 21, will be examined as an example.

Upon detecting engagement of the work device 100H, the main controller20 enters the combination operation shift mode described above. It thenreduces the power Pge supplied from the M/G inverter 25 to the travelinginverter 24 to, for instance, 30 kW and increases the power Pce suppliedfrom the converter 27 to the traveling inverter 24 to 20 kW as indicatedin the conceptual diagram presented in FIG. 21. Since a load Pem appliedto the engine 1 is the sum of a power generation load Pgm and a workload Ppm, the work load Ppm can be increased by an extent matching theextent to which the power generation load Pgm decreases incorrespondence to the decrease in the power Pge under assist control,thereby making it possible to allow a greater margin in the work load.

Thus, the engine 1 does not stall in the embodiment even if an addedwork load is applied while the wheel loader is traveling; for instance,as the operator operates the arm lever 57 to the “raise” position inorder to raise the arm 111 in the traveling state, as illustrated inFIG. 14. In the embodiment, the traveling motor 4 is assisted in stepS559 and, in addition, the target rotation rate Nt, having beencalculated based upon the pedal operation quantity during a leveroperation initial phase (pilot pressure p=p1) is corrected in step S576to a higher target rotation rate Ntc for the rotation rate of the engine1. A work load added during the lever operation initial phase (pilotpressure p=p1) in the traveling state does not come into effectimmediately. Namely, the rotation rate of the engine 1 is raised inadvance before the work load takes effect in the embodiment so as toreliably prevent the engine from stalling.

In reference to FIG. 19, the flow of the operation processing executedby the main controller 20 in the combination operation mode in which thetraveling drive device and the work device are both engaged in operationwill be explained. As FIG. 19 shows, as the processing in the travelingdrive device/work device combination operation mode starts, the maincontroller 20 reads the tilt control map for combination operation (seeFIG. 9(b)) in step S701, and then the processing proceeds to step S703.In step S703, the main controller 20 reads the engine control map forthe traveling drive device (see FIG. 5(b)), the electric powergeneration quantity map (see FIG. 5(a)) and the motor required torquemap (see FIG. 4), before the processing proceeds to step S705.

In step S705, the main controller 20 reads the engine rotation ratecorrection map for combination work (see FIG. 7) and the processing thenproceeds to step S706. In step S706, the main controller 20 reads theacceleration signal, the lever signal and the motor rotation ratesignal, and the processing proceeds to step S711.

In step S711, the main controller 20 calculates, based upon theacceleration signal and the motor rotation rate signal having been read,the motor required torque Tr by referencing the motor required torquemap (see FIG. 4), and then the processing proceeds to step S716. In stepS716, the main controller 20 calculates based upon the motor requiredtorque Tr, the electric power generation quantity Pe by referencing theelectric power generation quantity map (see FIG. 5(a)) and then theprocessing proceeds to step S721. In step S721, the main controller 20calculates, based upon the electric power generation quantity Pe, thetarget rotation rate Nt of the engine 1 by referencing the enginecontrol map for the traveling drive device (see FIG. 5(b)) and then theprocessing proceeds to step S726.

In step S726, the main controller 20 calculates a correction rotationrate ΔN based upon the lever signal by referencing the engine rotationrate correction map for combination work (see FIG. 7). The maincontroller 20 calculates a corrected target rotation rate Ntc of theengine 1 by adding a correction rotation rate ΔN having been calculatedas described above to the target rotation rate Nt of the engine 1, andoutputs the corrected target engine rotation rate Ntc of the engine 1 tothe engine controller 21. The engine controller 21 compares the actualrotation rate Na of the engine 1 with the corrected target rotation rateNtc, and controls the fuel injection device so as to adjust the actualrotation rate Na of the engine 1 toward the corrected target rotationrates Ntc.

In step S731, the main controller 20 outputs a drive signal, to be usedto convert AC power in the electric power generation quantity Pe havingbeen calculated in step S716 to DC power, to the M/G inverter 25. Thethree-phase AC power generated by the motor/generator 5 is converted toDC power by the M/G inverter 25, and the DC power in the electric powergeneration quantity Pe is provided to the traveling inverter 24.

In step S736, the main controller 20 outputs a drive signal, to be usedto convert the DC power provided from the M/G inverter 25 to three-phaseAC power, to the traveling inverter 24. In response, the travelinginverter 24 converts the DC power resulting from the DC conversion ofthe three-phase AC signal, provided from the motor/generator 5, at theM/G inverter 25, to three-phase AC power. The three-phase AC powerresulting from the conversion at the traveling inverter 24 is thenprovided to the traveling motor 4. As the three-phase AC power suppliedto the traveling motor 4 rotationally drives the traveling motor 4, arotational torque corresponding to the motor required torque Tr havingbeen calculated in step S711 is generated, and the traveling drivedevice 100D is driven with this rotational torque.

In step S741, the main controller 20 outputs a signal, to be used tocontrol the electromagnetic proportional pressure-reducing valve 64 inthe regulator 6, which is generated based upon the lever signal byreferencing the tilt control map for combination operation (see FIG.9(b)). During a combination operation, the tilt of the hydraulic pump 10increases over the range in which the pilot pressure p is equal to orgreater than the predetermined value p2.

In step S746, a decision is made as to whether or not the operator hasstepped on the accelerator pedal and the pilot pressure p is equal to orgreater than the predetermined value p1.

Upon making an affirmative decision in step S746, the processing returnsto step S706 and in this case, the traveling drive device/work devicecombination operation mode is sustained, whereas upon making a negativedecision in step S746, the processing exits the traveling drivedevice/work device combination operation mode and returns to step S1.

As described above, in the traveling drive device/work devicecombination operation mode, the tilt of the hydraulic pump 10 iscontrolled based upon the tilt control map for combination operation(see FIG. 9(b)) and also, the drive of the traveling motor 4 iscontrolled based upon the pedal operation quantity of the acceleratorpedal. In addition, the engine rotation rate is controlled based uponthe pedal operation quantity of the accelerator pedal and the leveroperation quantity of the arm lever 57. As a result, the operator isable to perform combination work utilizing both the traveling drivedevice 100D and the work device 100H with a high level of efficiency byachieving an optimal load distribution for the traveling drive device100D and the work device 100H through an operation of the arm lever 57and an operation of the accelerator pedal.

For instance, in the work mode in which the wheel loader travels as thearm 111 is being raised, as illustrated in FIG. 14, the operatoroperates the arm lever 57 to the electromagnetic hold position “raise”to achieve electromagnetic hold, and then higher priority can be givento drive of the hydraulic work system over drive of the traveling systemby easing off on the accelerator pedal. Since the speed with which thearm 111 is raised can be adjusted as the operator wishes, it can beensured with ease that the bucket 112 will be raised to the loadingheight by the time the wheel loader 100 reaches a point in front of thedump truck, and better work efficiency is thereby achieved.

In reference to FIG. 20, the flow of the operation processing executedby the main controller 20 in the work device solo operation mode will beexplained. As shown in FIG. 20, as the processing in the work devicesolo operation mode starts, the main controller 20 reads the tiltcontrol map for work device solo operation (see FIG. 8) in step S901 andthen the processing proceeds to step S903.

In step S903, the main controller 20 reads the engine control map forthe work device (see FIG. 6), before the processing proceeds to stepS906. In step S906, the main controller 20 reads the acceleration signaland the lever signal, and then the processing proceeds to step S921.

In step S921, the main controller 20 calculates, based upon the pilotpressure p, the target rotation rate Nt of the engine 1 by referencingthe engine control map (see FIG. 6) for the work device, and outputs thetarget rotation rate Nt of the engine 1 to the engine controller 21. Theengine controller 21 compares the actual rotation rate Na of the engine1 with the target rotation rate Nt and controls the fuel injectiondevice so as to adjust the actual rotation rate Na of the engine 1toward the target rotation rate Nt.

In step S941, the main controller 20 outputs a signal to be used tocontrol the electromagnetic proportional pressure-reducing valve 64 inthe regulator 6, which is based upon the lever signal by referencing thetilt control map for work device solo operation (see FIG. 8).

In step S946, the main controller 20 makes a decision as to whether ornot the accelerator pedal remains unoperated and the pilot pressure p isequal to or greater than p1.

Upon making an affirmative decision in step S946, the processing returnsto step S906 to sustain the work device solo operation mode, whereasupon making a negative decision in step S946, the processing exits thework device solo operation mode and returns to step S1.

As described above, in the operation mode referred to as the work devicesolo operation mode that is set when the work device 100H is beingdriven, and is thus in the engaged state, while, at the same time, thetraveling drive device 100D is not being driven, i.e., in thenon-traveling state, the main controller 20 references the tilt controlmap for work device solo operation (see FIG. 8). In this operation mode,the tilt of the hydraulic pump 10 is controlled based upon the leveroperation quantity at the arm lever 57. In addition, the main controller20 references the engine control map for the work device 100H (see FIG.6). On this occasion, the engine rotation rate is controlled based uponthe lever operation quantity of the arm lever 57. For instance, whenloading dirt or the like into a dump truck, as shown in FIG. 14, theengine 1 is controlled so as to achieve an engine rotation ratecorresponding to the lever operation quantity of the arm lever 57 evenif the accelerator pedal has not been operated. In this situation, thearm 111 can be raised at a speed corresponding to the lever operationquantity of the arm lever 57.

When a work vehicle, equipped with a torque converter known in therelated art, is engaged in loading work, the operator engages the workdevice 100H in operation with the engine rotation rate raised byoperating both the brake pedal and the accelerator pedal to the fullextent. For this reason, it is difficult to control the engine to adjustits rotation rate to an optimal rotation rate for the work mode. Incontrast, the engine rotation rate of the engine 1 can be controlled viathe control lever and thus an engine rotation rate optimal for the workmode can be achieved under the control executed in the embodiment.

While the flowcharts do not include the processing steps pertaining tothe SOC control, as mentioned earlier, the main controller 20 controlsthe engine 1, the M/G inverter 25, the traveling inverter 24, theconverter 27 and the like in correspondence to the vehicle operatingconditions, the charging rate and the like so as to ensure that thecharging rate of the power storage element 7 does not fall below apredetermined lower limit value and that it does not exceed apredetermined upper limit value. The vehicle operating conditions may beindicated by, for instance, the vehicle speed information and/or thepedal operation quantity of the accelerator pedal.

The following advantages are achieved with the wheel loader 100 in theembodiment described above.

(1) The rotation rate of the engine 1 is controlled based upon the leveroperation quantity and/or the pedal operation quantity in correspondenceto whether the work device 100H is in the engaged state or in thenon-engaged state and whether the traveling drive device 100D is in thetraveling state or in the non-traveling state. As a result, the operatoris able to adjust the load distribution for the traveling drive device100D and the work device 100H in correspondence to the current workstate and efficiently perform combination work by using the travelingdrive device 100D and the work device 100H with optimal loaddistribution. As a result, a hybrid work vehicle capable of operatingefficiently in correspondence to any work mode and is thus distinct froma work vehicle in the related art, in which the engine rotation rate iscontrolled entirely based upon the accelerator pedal operation quantity,can be provided.

(2) The traveling motor 4 is rotationally driven to drive the travelingdrive device 100D in correspondence to the pedal operation quantity ofthe accelerator pedal. The work device 100H in this wheel loader 100 isengaged in operation as the engine rotation rate and the tilt arecontrolled via the control lever and the control valve 11 is controlledvia the control lever. Thus, the work device 100H can be operatedthrough control lever operation alone without having to operate theaccelerator pedal.

The work device in a work vehicle equipped with a torque converter inthe related art is operated by raising the engine rotation rate throughan accelerator pedal operation and controlling the control valve via thecontrol lever. In contrast, the engine rotation rate can be controlledvia the control lever in the embodiment, which makes it possible toadjust the engine rotation rate more easily and ultimately reduce fuelconsumption, exhaust and noise.

(3) In a work vehicle equipped with a torque converter in the relatedart, the engine rotation rate is set to a high value in advance evenwhen there is no work load in effect in order to prevent an engine stallas the work load is applied in the traveling state. The embodiment isdistinguishable in that as the work load is applied in the travelingstate, the traveling motor 4 is assisted with power provided from thepower storage element 7, and the quantity of power generated by themotor/generator 5 is kept down by an extent corresponding to the assistto result in a reduced power generation load. Through these measures,stalling of the engine is prevented. Since this eliminates the need toset the engine rotation rate to a higher value in advance, the engine 1can be provided as a more compact unit and, at the same time, fuelconsumption, exhaust and noise in the wheel loader in the travelingstate can be reduced.

(4) In the embodiment, the engine rotation rate is increased once thepilot pressure p becomes equal to the predetermined value p1, which islower than the predetermined value pa, in any operation mode other thanthe work device solo operation mode. Since the engine rotation rate canbe raised before the work load is applied in the traveling state, theengine 1 can be very reliably prevented from stalling. Since the enginerotation rate does not need to be raised for the margin while the pilotpressure p is still under the predetermined value p1, fuel consumption,exhaust and noise can be minimized.

(5) In the excavation work mode, restrictions are imposed on the extentto which the tilt of the hydraulic pump 10 increases so as to givehigher priority to assuring good tractive force. Since the extent towhich the tilt of the hydraulic pump 10 increases is restricted evenwhen the control lever is operated to a significant extent to result inan increase in the output pressure of the hydraulic pump 10, the outputvolume is also kept down. As a result, the speed with which the arm 111is raised can be controlled to achieve an optimal speed that is not toohigh or too low. If the arm 111 is raised too fast in a work vehicleequipped with a torque converter in the related art engaged inexcavation work, the bucket 112 will not fully bite into the excavationtarget mound, i.e., dirt, and thus, only a small volume of dirt may bescooped into the bucket 112. If, on the other hand, the arm 111 israised too slowly during excavation work, the bucket 112 may be driventoo far into the dirt, resulting in slippage (spinning) of the frontwheels 113. In contrast, the embodiment makes it possible to generate agreat tractive force during excavation work by restricting the armraising speed under the control executed for the pump tilt, as indicatedin FIG. 9(a) and also outputting the required torque, which correspondsto the accelerator pedal operation quantity, via the traveling motor,and thus enables efficient excavation work.

The following variations are also within the scope of the presentinvention, and one of the variations or a plurality of the variationsmay be adopted in combination with the embodiment described above.

Variations

(1) While control under which the traveling motor 4 is assisted withpower provided from the power storage element 7 when the arm lever 57 isoperated in the traveling drive device solo operation mode has beendescribed in reference to the embodiment above, the present invention isnot limited to this example. Namely, the present invention may insteadbe adopted in control under which the traveling motor 4 is assisted withpower provided from the power storage element 7 when the acceleratorpedal is operated in the work device solo operation mode.

(2) While the traveling motor 4 is assisted with power provided from thepower storage element 7 with the timing with which the pilot pressure pis detected to be equal to p1 in the embodiment described above, thepresent invention is not limited to this example. For instance, theassist control may be executed upon detecting that the pilot pressure pis equal to pa. In addition, the tilt control map for combinationoperation may be set so that the tilt q is in proportion to the pilotpressure p over a range in which the pilot pressure p, indicated by theleaver signal, is pa˜pb, as indicated by the dotted line in FIG. 9(b).In such a case, upon detecting that the pilot pressure p is equal to pa,the output quantity, too, may be raised in correspondence to theincrease in the engine rotation rate.

(3) While the tilt control map presented in FIG. 9(b) and the enginerotation rate correction map presented in FIG. 7 are used both in thecombination operation shift mode and in the combination operation modein the embodiment described above, the present invention is not limitedto this example. Namely, the tilt control map and the engine rotationrate correction map used in the combination operation mode may bedifferent from the tilt control map and the engine rotation ratecorrection map used in the combination operation shift mode.

(4) While the decision as to whether or not the traveling drive device100D is being driven, i.e., whether or not the traveling drive device100D is in the traveling state, is made by detecting whether or not theoperator has stepped on the accelerator pedal and the decision as towhether or not the work device 100H is being driven, i.e., whether ornot the work device 100H is an the engaged state, is made by detectingwhether or not the lever has been operated in the embodiment, thepresent invention is not limited to this example. For instance, thetraveling state may be detected when, via the vehicle speed sensor 53, avehicle speed that is equal to or greater than a predetermined value hasbeen detected and the non-traveling state may be detected when, via thevehicle speed sensor 53, a vehicle speed less than the predeterminedvalue has been detected. In addition, the engaged state may be detectedwhen, via the pump pressure sensor 55, a pressure (output pressure)equal to or greater than a predetermined value has been detected and thenon-engaged state may be detected when, via the pump pressure sensor 55,a pressure (output pressure) less than the predetermined value has beendetected.

(5) While the tilt of the hydraulic pump 10 is controlled in theembodiment described above so that it starts to increase once the pilotpressure becomes equal to the predetermined value p2 greater than thepredetermined value pa (see FIG. 9(b)) during a combination operationother than excavation work, performed by engaging both the travelingdrive device 100D and the work device 100H in operation, the presentinvention is not limited to this example. For instance, control may beexecuted so as to limit the increase in the pump tilt only if the workload, in particular, is high during a combination operation. Namely, thepump tilt may be controlled based upon the tilt control map for workdevice solo operation presented in FIG. 8 when the work load is low andthe pump tilt may be controlled based upon the tilt control map forcombination operation presented in FIG. 9(b) only upon deciding that thepump output pressure detected by the pump pressure sensor 55 has becomeequal to or greater than a predetermined value.

(6) While the corrected target engine rotation rate Ntc (Ntc=Nt+ΔN) iscalculated by adding a correction rotation rate ΔN (see FIG. 7)calculated based upon the pilot pressure p to the target engine rotationrate Nt (see FIG. 5(b)) calculated based upon the electric powergeneration quantity Pe and a signal indicating the corrected targetengine rotation rate Ntc is output to the engine controller 21 duringcombination work including excavation work in the embodiment describedabove, the present invention is not limited to this example. The maximumvalue of the target engine rotation rate calculated by referencing bothof the engine control map for the traveling drive device and the enginecontrol map for the work device may be selected and a signal indicatingthe selected target rotation rate may be output to the engine controller21, instead.

(7) While the wheel loader 100 represents an example of the work vehiclein the description provided above, the present invention is not limitedto this example and may be adopted in another type of work vehicle suchas a forklift, a telescopic handler or a lift truck.

(8) While the power storage element 7 in the embodiment described aboveis constituted with a large-capacity electric double-layer capacitor inconsideration of installation space, cost, a charge/discharge responserate and the like, the present invention is not limited to this example.Namely, the present invention may be adopted in conjunction with a powerstorage element configured with secondary battery cells therein that canbe repeatedly charged/discharged, such as nickel cadmium batteries,nickel hydride batteries or lithium-ion batteries.

(9) The control units may adopt a configuration other than thatdescribed in reference to the embodiment. For instance, the functions ofthe engine controller 21 may be fulfilled in the main controller 20, andin such a case, the engine controller 21 will not be required. Asanother alternative, the various functions of the main controller 20 inthe embodiment may be distributed among microcomputers each installed incorrespondence to a specific function and, in such a case, themicrocomputers will replace the main controller 20.

(10) While the main controller 20 determines that the wheel loader 100is currently engaged in excavation work when the following conditionsare all satisfied in the embodiment described above, i.e., an arm raisecommand is output via the arm lever 57, a forward command for thetraveling drive device 100D is output via the forward/reverse changeoverswitch 51 and the angle of the arm 111 detected based upon the signalprovided from the arm angle sensor 54 is less than a predeterminedvalue, the present invention is not limited to this example. Forinstance, the main controller 20 may determine that the wheel loader 100is currently engaged in excavation work if a condition that the vehiclespeed, detected based upon the signal provided from the vehicle speedsensor 53, is less than a predetermined value and a condition that thepedal operation quantity, detected based upon a signal provided from theaccelerator pedal sensor 52, is equal to or greater than a predeterminedvalue, are both satisfied.

(11) While the traveling motor 4 is disposed upon the axis of the rearwheel-side propeller shaft 40R in the embodiment described above, thepresent invention is not limited to this example and the traveling motor4 may instead be disposed upon the axis of the front wheel-sidepropeller shaft 40F.

(12) The present invention does not always need to be adopted in astructure that includes only one traveling motor 4. For instance, it maybe adopted in a structure that includes a front wheel-side travelingmotor disposed on the axis of the front wheel-side propeller shaft 40Fand a rear wheel-side traveling motor disposed upon the axis of the rearwheel-side propeller shaft 40R, instead.

The present invention is not limited to the embodiment described aboveand allows for alterations and modifications to be made freely withoutdeparting from the scope of the invention.

The disclosure of the following priority application is hereinincorporated by reference: Japanese Patent Application No. 2012-31149filed Feb. 15, 2012.

The invention claimed is:
 1. A hybrid work vehicle, comprising: anengine; a hydraulic pump driven by the engine; a work device thatincludes a hydraulic cylinder and is driven with pressure oil providedfrom the hydraulic pump; a control lever that is operated to control thework device; a control valve that is provided between the hydraulic pumpand the hydraulic cylinder, the control valve being controlled by thecontrol lever and controlling drive of the hydraulic cylinder of thework device; a lever operation quantity detection unit that detects alever operation quantity of the control lever; an engagement statedetection unit that detects an engaged state and a non-engaged state ofthe work device; a motor/generator that is driven by the engine and thatgenerates AC power; a first inverter that converts the AC power,generated in the motor/generator, to DC power; a power storage unit thatexecutes charge/discharge control of electric power; a second inverterthat converts at least one of the DC power resulting from conversion inthe first inverter and the DC power output from the power storage unit,to AC power; a traveling motor that is driven with the AC powerresulting from conversion in the second inverter; a traveling drivedevice that is driven by the traveling motor; an accelerator pedal viawhich drive of the traveling motor is controlled; a pedal operationquantity detection unit that detects a pedal operation quantity of theaccelerator pedal; an engine control unit that controls the enginerotation rate based upon the pedal operation quantity detected by thepedal operation quantity detection unit; and a main controller thatcontrols the motor/generator and the traveling motor, wherein, while thehybrid work vehicle travels and when a pilot pressure output through thecontrol lever is in a lever operation initial phase in which the pilotpressure is greater than zero and less than a value on which the controlvalve is switched, the main controller drives the traveling motor withpower from the power storage and limits output of the AC power generatedby the motor/generator.